Machine type communication (MTC) is also referred to as machine-to-machine communication (M2M) or Internet of Things (IOT), and will become an important application in the communications field in the future. Future Internet of Things communication may cover fields such as smart metering, medical inspection and monitoring, logistics inspection, industrial inspection and monitoring, vehicle networking, smart community, and wearable device communication.
A typical cellular Internet of Things system is narrowband IOT (NB-IOT). An uplink system bandwidth and a downlink system bandwidth of the NB-IOT are generally 200 kHz, an operating bandwidth is 180 kHz, and each guard bandwidth on both sides is 10 kHz. An orthogonal frequency division multiplexing (OFDM) technology is used for downlink NB-IOT, and twelve subcarriers with a bandwidth of 15 kHz are multiplexed in a frequency domain. A single carrier frequency division multiple access (SC-FDMA) technology is used for uplink NB-IOT. SC-FDMA transmission is first performing DFT processing on a time-domain signal, mapping a processed signal onto a subcarrier of a corresponding frequency resource, and then modulating the signal in an OFDM modulation manner and sending a modulated signal. By means of such processing, a peak to average power ratio (PAPR) of a signal of SC-FDMA transmission is lower, which better helps implement a radio frequency component on user equipment (UE) such as a mobile phone.
The uplink NB-IOT can support two subcarrier spacings of 3.75 kHz and 15 kHz. When the subcarrier spacing of 3.75 kHz is used, UE supports only single-tone transmission (single-tone transmission), that is, a bandwidth of a time-domain signal of the UE is not greater than 3.75 kHz, and after DFT conversion, only one subcarrier with a subcarrier spacing of 3.75 kHz and in the OFDM modulation manner is occupied. When the subcarrier spacing of 15 kHz is used, UE may support both single-tone transmission (single-tone transmission) and multi-tone transmission (multi-tone transmission).
When uplink transmit power of UE is limited, a signal bandwidth of a subcarrier with a subcarrier spacing of 3.75 kHz is only ¼ of a bandwidth of a subcarrier with a subcarrier spacing of 15 kHz. Therefore, a power spectral density of a transmitted signal of the subcarrier with a subcarrier spacing of 3.75 kHz is four times that of the subcarrier with a subcarrier spacing of 15 kHz, and better anti-interference and anti-path loss performance are gained. Therefore, the subcarrier with a subcarrier spacing of 3.75 kHz is more applicable to UE with poor coverage, for example, UE on a cell edge and even in a basement.
When a 3.75 kHz uplink subcarrier is embedded and deployed in a bandwidth resource of legacy Long Term Evolution (LTE), the following problems exist. On one hand, after transmission with an uplink subcarrier spacing of 3.75 kHz is introduced, a suitable time unit needs to be defined to define a physical resource block. Generally, the time unit is referred to as a subframe. A subframe time length and a subframe structure need to be defined to make transmission efficiency of the NB-IOT as high as possible. That is, as many uplink OFDM symbols as possible are transmitted in each subframe time length.
On the other hand, mutual impact between 3.75 kHz uplink deployment and legacy LTE needs to be minimized. For example, an uplink channel sounding reference signal (SRS) of UE in legacy LTE cannot be affected. In addition, because coverage of an NB-IoT user that uses uplink 3.75 kHz is generally poor, interference from a channel sounding reference signal of legacy LTE may cause relatively large impact on SC-FDMA transmission of uplink 3.75 kHz, which should be avoided. In legacy LTE, a base station may configure a piece of information srs-SubframeConfig in cell-level system broadcast information, where the information indicates a subframe pattern (subframe Pattern) in which an SRS can be sent, and UE in the cell may send an SRS in only subframes indicated by the SRS subframe pattern. In legacy LTE, because UE may send an SRS on only the last OFDM symbol of the indicated subframes, when a frame structure of the NB-IOT is being designed, such a factor may be considered, to avoid mutual interference between an OFDM symbol that is sent by an NB-IOT terminal and that has an uplink subcarrier spacing of 3.75 kHz and an SRS that may be sent by a legacy LTE terminal.
Therefore, the foregoing two factors need to be considered for a 3.75 kHz uplink frame structure in the NB-IOT.